Hi mangtin,
Just to add to Jonathan's answer below, we followed the same scores as you get in the word game Scrabble. Jonathan's totally right, that the most difficult, and less commom letters to use get you more points. At the beginning of Sushi Spell, you can click on the HELP button and you will see exactly how many points each letter gives you. I hope you're enjoying playing Sushi Spell!
Jo (LearnEnglish Teens Team)

Well, it's about how common the letter is in English words. E is a very common letter and it's easy to make words with E in them. So, E is only 1 point. But Z is not very common. Not many words have Z in them, so it's very difficult to use this letter. That's why you'll get 10 points if you can do it.